Passive Radiator Sub

Let' talk subwoofers.

I have noticed a trend in smaller subs that are utilizing passive radiators. Def Tech, Polk, Klipsch, Boston Acoustics as well as many other companies have been producing and/or are introducing subs with passive radiators.

I like the sound that passive radiator subs create and I like their size. I wouldn't characterize their sound as "tight", I would describe what I have heard as defined.

Passive radiator subs are smaller which is beneficial with your decor, placement and acceptance with the Mrs. Also, with dual sub layouts I believe they can be more versatile. I believe one advantage is the design that incorporates a front or down firing woofer with the radiators that are on each side. I have listened to a few of these designs that were placed along a side wall and was very impressed with their sound projection. I understand the discussion that low notes are non directional, but what about the sound waves that you can feel and are ambient?

I would love to see an Outlaw Passive Radiator sub and would be very interested in everyone's thoughts on the design and experience with them.

Passive radiators do give you a little boost in bass volume, though they don't do much for extension, and to me, don't produce as tight a sound.

I was able to listen to a Boston Acoustics RPS 1000 which has a front firing 10 inch woofer and two side mounted 8 inch passive radiators. During rock oriented music, I really enjoyed it's performance. When blues music was played I thought that the Klipsch 12 inch ported sub sounded better. I also thought that the 12 inch ported was more appealing during some movie scenes with explosions, but the Boston Acoustic sub was more impressive for a majority of the movie. I believe this could possibly be the way the subs were tuned.

So maybe your thought that there is boost in bass volume at certain frequencies is the reason, but I am wondering if a passive radiator design might be more beneficial at lower volumes.

Evidently Dr. Hsu would not agree with me:

http://forum.hsuresearch.com/showthread.php?t=44073

I have two kinds of subs, each in a different system. One type is a 'small cube' with active and passive transducers back-to-back. The other type is the Outlaw LFM-1 EX, in my case with ports rigged for extension rather than amplitude.

(Before continuing ... I have found that the sense of 'tightness' (if you mean quick response) of most good subwoofers has more to do with the way they are blended with the rest of the system, including system delay setup, than strictly on their own IF this is the case: the crossover point is no higher than 80Hz and increasing attenuation above 80Hz is in a rather steep curve. How 'quick' should a subwoofer sound if it's not reproducing anything significant at 'quick' frequencies? Of course there shouldn't be significant lag either.)

Both types of subwoofers appear to accomplish something similar in different ways because, at lower frequencies, the active transducers need 'help'.

In the case of the 'small cube', the passive radiator is somewhat 'tuned' to the lower end of the sub's frequency response curve, hardly having any response in the upper octave of the subwoofer's range. On the other hand, the passive transducer sees a huge increase in excursion when frequencies at the low end of the lower octave are present. Basically, as the active transducer's frequency response starts to fall off, the passive transducer plays an increasing larger role.

In the case of the ported enclosure, the active transducer also needs 'help' at the lower end of the frequency range. In this case, the enclosure is 'tuned' to assist the active radiator as frequencies head lower, with a mix of resonance behind it to help the active transducer maintain a larger low-frequency excursion, and acoustic energy exiting any ports.

Which am I more happy with? Hard to say because they're in different rooms and the behavior of sound waves in a room plays a very significant role in what the listener hears. The Outlaws were lower cost, down to about 1/2 or 1/3 the cost of the 'small cube' when one is pricing each type as new. If such savings are preferred, and one is not trying to 'hide' the subwoofer, I heartily recommend subwoofers of the Outlaw LFM-1 EX type. For the size though, the small cube's passive radiator 'goes lower' than the ported cabinet of the LFM-1 EX, even at the LFM-1 EX's size. If I send a 16Hz signal to the 'small cube', the active radiator shows a greatly reduced extension, but the passive radiator is moving quite well - I don't directly hear the sound, but stuff in the room vibrates and, depending on how I'm standing, my blue jeans (pants, not cables) 'wobble' on my legs. Very little recorded material has intentional frequency response anywhere near that low, but if you want to 'feel it' without cabinets larger than the LFM-1 EX and have the extra coin to spend (or patience and maybe many months' time to hunt for used bargains*), and if you want to keep the size as small as possible, I can recommend the 'small cube' type as well.

* how bestbang4thebuck went about it :-)

Hey guys. Been gone a while: Got a new job that's keeping me too busy to lurk on the forum

Ported and passive radiator subs are both built on the same concept. Look up Helmholz Resonator. It's the same concept as blowing on the neck of a bottle: The smaller the volume the higher the pitch.

The ported sub relies on the mass and springiness of the air in the port while the passive radiator relies on the mass and springiness of the passive radiator. The passive radiators are cool becasue you can control the mass and springiness directly instead of indirectly by changing the volume of the port.

So you should be able to get similar performance out of either system but you have to make different tradeoffs. I would image that any differences you're hearing are due to the individual designs and not whether one technology is better than the other.

Having said that: One advantage of passive radiators is that you don't have to worry about port noise.

So would it be accurate to say that it's about moving air? A ported sub has the ports for moving air, the passive radiators are used instead of ports to move air. Where does that put the sealed subs?

All but one of the DefTech subs have passive radiators. Klipsch, Polk, Boston Acoustics, Sunfire are all manufacturing passive radiator subs as their main product line of subs. Is it because you have more control over the tuning?

I would think that a passive radiator would provide a little more than a port would with respect to sound because it is another cone, although not active.

As far as Outlaw goes, I don't see them getting into the "who can build the biggest sub" fever that seems to be the rage with all of the other internet direct companies. Just how many of those can you sell? Why not play on the field with the mass market guy's, all of their subs start around $1500.00. Outlaw can definitely build a unit far superior at a much more competitive price and it would sync really well with their LCR speakers which are sealed.

Unless of course, sealed trumps them both.

It's more about the resonance set up by the port or passive radiator interacting with the driver and the box. You can think of them as low frequency amplifiers for your main driver. As your driver starts to roll off on the low end the resonator starts to pick up giving you extended low end. The trick is to tune the resonator so that it ramps up at the right point where you driver ramps down so you don't end up with humps in your frequency response.

Of course some people like those humps in the frequency response because it maske your sub sound louder. for example Outlaw has the LFM-1 EX with two ports: plug one port and you get "Bass Extension". Leave them both open you get "Max Output". Opening both ports moves the resonance higher so that it adds to the main driver instead of complementing it and extending the low end.

Sealed subwoofers do not rely on resonance to boost their output. They are typically considered the most "musical" (Jimna can tell you what that means. I just know how to build the darn things:) of the subwoofer designs because the fequency responses can be kept pretty flat. However they tend to be big and require a lot of power.

I don't understand H-PAS technology and how that relates to the above so I'll just end here.

Bestbang4thebuck stated:

"If I send a 16Hz signal to the 'small cube', the active radiator shows a greatly reduced extension, but the passive radiator is moving quite well - I don't directly hear the sound, but stuff in the room vibrates and, depending on how I'm standing, my blue jeans (pants, not cables) 'wobble' on my legs".

Is that what would be considered "visceral impact" or "slam"? What creates this? The ports or Radiators? Or is it the low frequency? If it is the ports or radiators, then would it be correct to assume that a sealed sub would not.

It's an example of what I described above with the driver (active radiator) rolling off and the passive radiators, which are tuned to resonate at the lower frequencies, picking up the vibration from the driver and amplifing it. Call it what you want but the effect is casued by the low frequencies at higher SPL (Sound pressure level). A sealed sub can do this too. I have plans on the drawing board for a sub with a 15" driver in a 5 cubic foot sealed box with either a 500 or 1000 watt amplifier. If I ever get around to building it, it will make a nice end table. It'll do 16Hz no problem but there's no way you'd ever call it a "small cube"

I prefer sealed box 10 out of 10 times a day. Its more wattage to achieve equal SPL's but it just sounds better/tighter, and that is all that matters to me.

This seems to be the consensus with the people that I have spoke with who have had a chance to listen to a well built sealed sub. What's puzzling to me goes back to the first post, is why we are seeing a trend towards passive radiators with the large manufacturers? I would think that it would be more beneficial cost and profit wise, to manufacture and sell sealed subs as opposed to passive radiator designs.

Could it be a size decision? Passive radiator subs can utilize a smaller enclosure where a good sealed sub would have to house a 12 or 15 inch woofer which would require a larger box?

I think its because you can get equal SPL out of a ported sub with less wattage than a sealed version. That means the amp and driver of a seal box need to be much beefier which is much more expensive than just designing a ported sub. And like everything in todays fast food drive through mentality world, cheaper sells more than quality.

All a guess on my part though, YMMV.

Don't forget good marketing. The best technical solutions don't always win out when the marketeers get involved.

But yes, for a given SPL the ported design will be smaller and require less power. But since nothing is free your going to pay for it in other areas.